Ok. I've come up with a guitar riff thats in drop d tuning. It's a metal riff and it sounds awesome. the notes are open, 2nd fret, 3rd fret, 5th fret, 7th & 8th fret. Its a simple chord progression with some simple phrasings and pedal toning. I think it might be E harmonic minor but I can't be sure. If i'm right than its in the key of E minor. But I know I don't have to use just one scale. I know that other metal songs use several different scales as long as they are in the same key but I think I might be missing something. Any tips would be great.

Which frets are played is useless to us if we don't know which string it is on. But the frets you are saying you use do not imply any sort of harmonic minor scale. If the riff ended going up to the 11th fret then it would be harmonic minor but these notes are the first 6 notes of a minor scale. The harmonic minor scale has the completely same first 6 notes as does the natural minor. So as I said before, I don't know which string you will play this on (you didn't tell) but if you play it on the A string, then it will be in the key of A minor. If you have drop D tuning, you will be in the E min key only if you play this on the high E string, because the low E string is detuned to D. I think you get it y now...

...several different scales as long as they are in the same key...

Ermm, yeah. A song being in the key of (something) means it is using the notes from the (something) scale. The key of E major can only utilize the E major scale, and so on. If it is a minor key, then you can use all three variants of the minor scale, but it will still be the same minor scale, not a different one.
That doesn't mean you can't use some notes that are outside of the scale to add flavour, but if you use these too much, then your solo/song will sound off key, one can only associate it with the chromatic scale. For using "off-key" notes you can try combining the blues scale with the minor scale (basically adding an augmented fourth, that means if you are in A minor you add a D# to the other notes), it adds a bit more suspense or a sinister feel to the melody, depends on how you use it.
You can also utilize a key change in some sort of interlude in a song, but that's a bit more advanced
At first I recommend you stick to the scale and make excursions into chromatics only when you become more experienced or you will just sound messy.